1. Field of the Invention
The present invention relates to a multi-cylinder vertical engine including a plurality of cylinders each having a generally horizontal axis and juxtaposed vertically in a cylinder block, and to an outboard engine system including a vertical engine mounted thereon.
2. Description of the Related Art
A surface of a piston slidably received in each of cylinders of an engine, which faces a combustion engine, is exposed to a high temperature and hence, it is desirable that a low-temperature oil is brought into contact with a rear face of the piston to cool the piston. There is a vertical engine known from Japanese Patent Application Laid-open No. 2001-200711 in which a portion of an oil passed through an oil passage formed in a crankshaft to lubricate sliding portions of a crankpin and a big end of a connecting rod is injected from an oil injecting groove provided in the big end to the rear face of the piston to cool the piston.
In a vertical engine including a crankshaft disposed in a generally vertical direction, an oil in a mist state in a crank chamber is introduced into each of cylinders to cool a piston from behind, but the concentration of the oil mist is dilute in an upper cylinder and dense in a lower cylinder by gravitation. Also, the oil supplied to portions to be lubricated of the engine flows from a higher position to a lower position to return to an oil pan, and hence those of the pistons, the cylinders, connecting rods and the like, which are located at lower places, are brought into contact with a large amount of the oil and cooled effectively, that is, the cooling conditions are more severe for the upper piston and milder for the lower piston.
In the above-described prior art engine, however, the following problem is encountered: The oil is injected equally to the rear faces of all the pistons to cool the pistons and hence, if the amount of oil injected is determined based on the requirement of the upper piston, the amount of oil injected is rather excessive in the lower cylinder for the above-described reason. Correspondingly, the unnecessary oil is injected, resulting in an increase in required amount of the oil. Moreover, the prior art engine is a vertical in-line 2-cylinder engine and for this reason, the vertical dimension of the crank chamber is smaller and hence, the localization of the oil mist is less, and an increase in required amount of the oil is smaller as a whole. In an engine including three or more cylinders, however, any measure is demanded in order to decrease the required amount of the oil.